Dark Matter
In astronomy and cosmology, dark matter is a currently-undetermined type of matter which accounts for a large part of the mass of the universe, but neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly seen with telescopes. Dark matter is presumed to constitute 83% of the matter in the universe and 23% of the mass-energy. Dark matter came to the attention of astrophysicists in recent decades due to discrepancies between the mass of large astronomical objects determined from their gravitational effects, and mass calculated from the "luminous matter" they contain; such as stars, gas and dust. It was first postulated by Fritz Zwicky in 1933 to account for evidence of "missing mass" in the orbital velocities of galaxies in clusters. Subsequently, other observations have indicated the presence of dark matter in the universe, including the rotational speeds of galaxies, gravitational lensing of background objects by galaxy clusters such as the Bullet Cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. According to consensus among cosmologists, dark matter is hypothesized to be composed primarily of a new, not yet characterized, type of subatomic particle. The search for this particle, by a variety of means, is one of the major efforts in particle physics today.Though the existence of dark matter is generally accepted by the mainstream scientific community, some alternative theories have been proposed to explain the anomalies that dark matter is intended to account for, without hypothesizing dark matter. Baryonic and nonbaryonic dark matter A small proportion of dark matter may be baryonic dark matter: astronomical bodies, such as massive compact halo objects, that are composed of ordinary matter but which emit little or no electromagnetic radiation. The vast majority of dark matter in the universe is presumed to be nonbaryonic, and thus not formed out of atoms. It is also presumed that it does not interact with ordinary matter via electromagnetic forces; in particular, dark matter particles do not carry any electric charge. The nonbaryonic dark matter certainly includes neutrinos, and possibly hypothetical entities such as axions, or supersymmetric particles. Unlike baryonic dark matter, nonbaryonic dark matter does not contribute to the formation of the elements in the early universe ("Big Bang nucleosynthesis")and so its presence is revealed only via its gravitational attraction. In addition, if the particles of which it is composed are supersymmetric, they can undergo annihilation interactions with themselves resulting in observable by-products such as photons and neutrinos ("indirect detection"). Nonbaryonic dark matter is classified in terms of the mass of the particle(s) that is assumed to make it up, and/or the typical velocity dispersion of those particles (since more massive particles move more slowly). There are three prominent hypotheses on nonbaryonic dark matter, called Hot Dark Matter (HDM), Warm Dark Matter (WDM), and Cold Dark Matter (CDM); some combination of these is also possible. The most widely discussed models for nonbaryonic dark matter are based on the Cold Dark Matter hypothesis, and the corresponding particle is most commonly assumed to be a neutralino. Hot dark matter might consist of (massive) neutrinos. Cold dark matter would lead to a "bottom-up" formation of structure in the universe while hot dark matter would result in a "top-down" formation scenario. One possibility is that cold dark matter could consist of primordial black holes in the range of 1014 kg to 1023 kg. Being within the range of an asteroid's mass, they would be small enough to pass through objects like stars, with minimal impact on the star itself. These black holes may have formed shortly after the big bang when the energy density was great enough to form black holes directly from density variations, instead of from star collapse. In vast numbers they could account for the missing mass necessary to explain star motions in galaxies and gravitational lensing effects.